This invention relates to a flexible detection label for an electronic detection system, comprising a thin, platelike flexible carrier having on both sides elements made from flexible material, jointly forming at least one resonant circuit.
Such labels may be used, e.g. in a shop-lifting detection system, as described e.g. in U.S. Pat. No. 4,686,517 (Nedap). The resonant circuit includes at least one coil and at least one capacitive element, jointly forming the operating frequency of the label.
The resonant circuit may consist e.g. of a flexible coil of metal foil on a sheet of paper or on a foil of synthetic plastics material, functioning as a carrier. The coil may be made e.g. by etching from aluminum foil or by means of printing techniques or any of the other techniques known therefor. The coil is connected to a capacitor whose one electrode is disposed on the same surface of the carrier as the coil, and is connected to one end of the coil. The other electrode of the capacitor is provided on the other surface of the carrier and is connected to the other end of the coil by means of an interconnection through the carrier or around the edge of the carrier.
U.S. Pat. No. 4,694,283 to Reeb discloses a flexible identification label consisting of a conductor configuration provided on one of the surfaces of a carrier of paper, synthetic plastics material or cardboard, said configuration extending on both sides of at least one fold line and having a conductor track bridging the fold line. By doubling up the carrier with the conductor configuration thereon along the fold line, there is produced an assembly of two opposite sub-configurations, each forming a coil having a capacitor face on at least one end. In the folded condition, the capacitor faces of the two sub-configurations form one capacitor. The two sub-configurations are electrically interconnected by the conductor track bridging the fold line.
A drawback of this known label is that the electrical properties of such a folded label depend strongly on the exact location of the fold line. Consequently, the fold line should be defined as accurately as possible, which is effected by means of perforations or ridges. However, this entails the risk that the conductor track bridging the fold line is interrupted entirely or partly. Furthermore, during the further handling of such a label, a conductor track bridging an outer edge is rather vulnerable.
U.S. Pat. No. 4,658,264 to Baker describes various techniques for defining the fold line of a folding label as accurately as possible. However, the above described drawbacks apply just as well to the labels described in that publication.
It is observed that both the Reeb and Baker patents aforementioned describe embodiments wherein the sub-configurations on opposite sides of the fold line are not connected by a conductor track bridging the fold line. In the folded condition, the two sub-configurations are then coupled exclusively capacitively. In the known configurations, however, this capacitive coupling is defined substantially by the conductor tracks forming the coils. Sometimes, even, the parasitic capacitive coupling of the conductor tracks forming the coils is used only. Needless to say that in that case, even a very small deviation during folding may result in a substantial variation in the resonate frequency of the label. Moreover, the coil and the capacitive element can then no longer be designed independently from one another in an optimal manner, given the desired resonate frequency.
The Reeb patent shows an embodiment wherein opposite spiral conductor tracks have inner ends enlarged to form a capacitor electrode. As a result, true, the freedom of design is somewhat increased, but the effective coil surface area is reduced.
The major problem of the construction of these flexible labels is to ensure that the effective coil area is as large as possible with slight outer dimensions, as well as to achieve that the quality factor Q of the circuit is as high as possible. The effective coil area defines the coupling factor with the transceiver coil and hence the detection sensitivity.
The loss resistance in the coil, i.e. the resistance of the tracks, defines substantially the quality factor Q of the resonant circuit.
A high quality factor Q requires a low resistance of the tracks and hence a large track width. Broad tracks, however, result in a smaller effective coil area.
It is therefore an important object of the present invention to provide an improved detection label.